Fault tolerant linear actuator

ABSTRACT

In varying embodiments, the fault tolerant linear actuator of the present invention is a new and improved linear actuator with fault tolerance and positional control that may incorporate velocity summing, force summing, or a combination of the two. In one embodiment, the invention offers a velocity summing arrangement with a differential gear between two prime movers driving a cage, which then drives a linear spindle screw transmission. Other embodiments feature two prime movers driving separate linear spindle screw transmissions, one internal and one external, in a totally concentric and compact integrated module.

The U.S. Government may own certain rights in this invention pursuant tothe terms of the U.S. Department of Energy grant numberDE-FG04-94EW37966. This application claims priority to U.S. ProvisionalPatent Application Ser. No. 60/386,661, filed Jun. 5, 2002.

BACKGROUND OF THE INVENTION

The present invention relates generally to electro-mechanical actuators,and specifically to a linear actuator having improved fault toleranceand positional control.

A number of approaches have been developed to manipulate the linearposition of an object or device through the use of an actuator. Linearactuators are pervasive where the movement of very large loads isrequired. Linear actuation has traditionally been met by the use ofhydraulic and pneumatic cylinders. Electromagnetic actuators are known,however, to provide increased performance in many aspects as compared toeither hydraulic or pneumatic cylinders.

One drawback to the use of electromagnetic actuators is a certain degreeof increased complexity, giving rise to increased concern over thereliability of such devices. Accordingly, certain electromagnetic linearactuators have incorporated fail-safe mechanisms of one type or another.As an example, U.S. Pat. No. 4,289,996 discloses a powered linearactuator having dual closed loop servo motor systems driving a screwjack. The dual motors drive the screw jack through differential gearingand each has an armature lock which functions automatically if a motorcircuit fails thereby enabling the other motor to continue driving theactuator alone. Potentiometer feedback is applied to dual erroramplifiers or polarized relays that compare the feedback position signalwith the input command signal and drive separate motor energizationchannels.

U.S. Pat. No. 5,865,272 discloses a linear actuator having an outputshaft having a pair of driven wheels mounted thereon. One of the drivenwheels is rotatably mounted in a fixed plane and has a drive nut for anassociated thread on the output shaft. The other drive wheel isrotatably fixed to the output shaft. An input shaft is in a side-by-siderelationship with the output shaft and adapted to be rotated by asuitable power source. The input shaft provides a drive wheel for eachof the driven wheels, with the ratio between each drive and driven wheelset being chosen to rotate the driven wheels at different speeds in thesame rotational direction and thereby produce a controlled axialmovement of the output shaft in a direction depending upon the relativerotation of the driven wheels. A fail-safe arrangement is provided inthe form of a clutch between the drive wheels of the input shaft, aback-drive for the output shaft, and biasing means for affecting aback-drive.

U.S. Pat. No. 5,957,798 discloses an electromechanical actuator having alinear output for moving an external load, the actuator having at leasttwo drive motors, a synchronizer connected to the outputs of the drivemotors, a differential mechanism combining the outputs of the drivemotors, and a quick release mechanism connected to the differentialmechanism and the actuator output. The quick release mechanism releasessupport of the external actuator load in response to an internalactuator jam and maintains support of the external actuator load inresponse to an external actuator overload.

U.S. Pat. No. 6,158,295 discloses a linear actuator including a housing,a spindle rotatable in both directions, a threaded nut driving a pistonrod, and a motor capable of driving the spindle through a transmission.A disengagement unit is arranged in the transmission for interruptingthe connection between the motor and the spindle in case of operationalfailure, such as overloading of the spindle. The disengagement unitcomprises a braking device adjustable with respect to the actuatorhousing to cooperate with a coupling device for control of therotational speed of the spindle when it is disengaged from the motor.

Although each of these designs provides certain advantages, none ofthese designs provides a fully fault-tolerant linear actuation solutiontotally suitable for use in applications where life or safety is atrisk. Each of these designs has its drawbacks, as will be appreciated bythose of skill in the art. For example, as noted above, in anyapplication in which a mechanical device, such as an actuator, isemployed to perform a function, there is the potential and the risk offailure of the mechanical device and attendant loss of functionality. Incertain situations, such failure may have only minor consequences.Wherever actuators are employed in applications in which life or safetyare at risk, however, the consequences are much more severe. Inhigh-stakes applications, such as the control of an aircraft controlsurface, disengagement of the actuator from the applied load is simplynot an acceptable approach. Similarly, locking up the actuator with abrake would generally not be an acceptable approach in such anapplication. Accordingly, there is an unmet need to prevent sudden orcatastrophic failure in the linear actuators employed.

Although electromechanical solutions offer definite advantages over thelower-technology hydraulic and pneumatic solutions often used intraditional linear actuation applications, the rugged simplicity of thefluid cylinder has made it tough to beat from a cost and reliabilitystandpoint. Further, it is known that single point failures frequentlyoccur in electromagnetic linear actuators. Where a linear actuator issusceptible to loss of function from a single point failure, theactuator could completely fail to operate in the event of such afailure. As noted above, this is an unacceptable situation in manyapplications.

SUMMARY OF THE INVENTION

The present invention solves the problems associated with current linearactuators. For example, in various embodiments, the systems of thepresent invention overcome the risk of failure by incorporating featuresenabling them to continue to operate under a partial or total fault onone side of a dual system. Thus, the present invention provides, incertain embodiments, fault tolerant duality in a compact, concentric,fully integrated module. This compactness and integration does not existin any existing designs.

In accordance with one aspect of the present invention, a fault tolerantlinear actuator is provided that incorporate velocity summing, forcesumming, or a combination of the two. In one embodiment, the inventionoffers a velocity summing arrangement with a differential gear betweentwo prime movers driving a cage, which then drives a linear spindlescrew transmission. This embodiment is reconfigurable, but since it hasonly one transmission, it does not eliminate all possible single pointfailures. A second embodiment features two prime movers driving separatelinear spindle screw transmissions (one internal and one external) in atotally concentric and compact integrated module. This system has nosingle point failures, which is desirable where failure would result inloss of life or high cost. A third embodiment uses two rotary actuatorsdriving acme screws in place of the linear spindle screw transmission tomake a very rugged high force system. A fourth embodiment is a forcesumming linear actuator based on a dual set of linear spindle screwdrives summing forces through two clutches at the output attachmentplate. A fifth embodiment uses an intermediate gear train between theinput prime movers and the output spindle screws in order to betterbalance the torque/speed ratios and to enable a significantly highermotor speed than in the second embodiment. This two-stage reduction alsoallows for a significant reduction in the weight of the actuator.

The development of certain technologies makes it possible for theelectromechanical actuators of the present invention to surpass theperformance of prior known designs in essentially every aspect ofperformance. As an example, the commercial availability of the rollerspindle screw transmission is a significant step forward in performance.As another example, the development of modern highly-integrated circuitsallows for increases in performance and reductions in cost at the sametime. Using these and other technologies, the present invention not onlyoffers high load capacity, it also offers very long life, highprecision, and high velocity in a compact configuration and thepotential for a high level of actuator intelligence.

Intelligence within the actuator itself makes it possible to balanceoperational priorities (speed, load, precision, smoothness, etc.) inreal time. Intelligence within the actuator permits the system of thepresent invention to be highly fault tolerant. This fault tolerancedepends on a full awareness of all the performance capabilities of theactuator in real time. This awareness requires access to a wide spectrumof sensors, each generating data quantifying performance criteria usedto judge the actuator's operation. Depending on the application, theseperformance criteria may be prioritized to meet in-situ operationalgoals. Here, the principal goal is to maintain operation under a fault.Depending on the operational requirements, the output of a faulty primemover in an actuator may be quantified and used as a basis totemporarily raise the performance of the one or more fully-operationalprime movers in order to make up for the loss of performance from thefaulty prime mover. Alternately, the faulty prime mover may be takencompletely out of service by braking it and “limping home” using theremaining prime movers.

The teachings of the present invention may be employed in anyapplication in which there is the potential for loss of life, a need topreserve a long mission in harsh environments without possibility ofrepair, or a potential for high cost resulting from sudden failure. Thislayered control should combine to give more precise operation undersignificant load disturbances.

Those skilled in the art will further appreciate the above-mentionedadvantages and superior features of the invention, together with otherimportant aspects thereof upon reading the detailed description thatfollows in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For more complete understanding of the features and advantages of thepresent invention, reference is now made to the detailed description ofthe invention along with the accompanying FIGURES.

FIG. 1 depicts an isometric cutaway view of a velocity-summingfault-tolerant linear actuator according to one embodiment of thepresent invention;

FIG. 2 depicts an isometric cutaway view of a velocity-summingfault-tolerant linear actuator according to a second embodiment of thepresent invention;

FIG. 3 depicts an isometric cutaway view of a dual fault-tolerant linearmodule based on a combination of rotary actuators;

FIG. 4 depicts an isometric cutaway view of a force-summingfault-tolerant linear actuator; and

FIG. 5 depicts an isometric view of a velocity-summing fault-tolerantlinear actuator with two-stage transmissions.

DETAILED DESCRIPTION OF THE INVENTION

Although making and using various embodiments of the present inventionare discussed in detail below, it should be appreciated that the presentinvention provides many inventive concepts that may be embodied in awide variety of contexts. The specific aspects and embodiments discussedherein are merely illustrative of ways to make and use the invention,and do not limit the scope of the invention.

FIG. 1 depicts an isometric cutaway view of a velocity summing faulttolerant linear actuator 100 according to one embodiment of the presentinvention. Actuator 100 provides a dual set of prime movers 102 and 104operating through a differential gearset 106, which then drives a cage108 containing two sets of spindle screw drives 110 and 112 operating ona single linear output screw 114. Fault tolerant linear actuator 100 isfault tolerant up to the differential gearset 106, e.g., either primemover 102 or 104 may be disabled (e.g., braked) and the remaining primemover 102 or 104 may still operate.

The trunnion 116 as part of the outer shell 118 provides one attachmentto the environment with the other attachment being on the linear outputscrew 114. The dual prime movers 102 and 104 are arranged in asymmetrical layout. Prime mover 102 incorporates field coil cylinder 120and armature 122. Prime mover 104 incorporates field coil cylinder 124and armature 126. Prime movers 102 and 104 are mounted on rotary needlebearings 128 and 130, respectively, and drive multiple centraldifferential planetary gears 132 mounted on bearings 134 in planetarycage 136 supported by planetary cage needle bearing 138.

The planetary cage 136 also contains the planetary screws 140 and 142supported by thrust bearings 144 and 146. The planetary cage 136 as aunit is supported by principal thrust bearings 148 and 150 in the outershell 118 of the actuator 100. The end plates 152 and 154 of theactuator 100 are fixed to the shell with machine bolts 156.

Depending on the application, actuator 100 may be designed to providevarying types of service, e.g., light, medium, or heavy-duty service.Actuator 100 is dynamically reconfigurable in real time. Should oneprime mover (e.g., 102 or 104) lose torque capacity past a certainlimit, the remaining prime mover (e.g., 104 or 102) may beinstantaneously raised to greater than 100% of its normal torquecapacity to maintain the normal level of performance for the actuator100. Sensor systems, operational criteria, and performance histories maythen be used to monitor the performance of actuator 100 relative to itsreduced performance envelope.

FIG. 2 depicts a velocity summing linear fault tolerant actuator 200having no single point failures. Fault tolerant actuator 200incorporates a pair of rotary prime movers 202 (1) and 204 (2), that mayeither be, e.g., BDCM or SRM-type, motors, driving a pair of linearspindle screw transmissions 206 and 208 acting on an external screwshaft 210 and an internal screw cylinder 212. Fault-tolerant actuator200 incorporates an inner motion frame 214 that travels along both theexternal screw shaft 210 and the internal screw cylinder 212. Innermotion frame 214 also contains the two rotary prime movers 202 and 204and their associated planetary screws 216 and 218. Inner motion frame214 is prevented from rotation on these screws with the use of linearcross-roller bearings 220 and 222. The length and placement of thesecross-roller bearings 220 and 222 will be dependent on the strokerequirements of the application.

As seen in FIG. 2, external screw shaft 210 functions as the outputshaft for the fault-tolerant actuator 200 while the outer shell 226,which contains the internal screw cylinder 212, also incorporates theinput attachment 228. Linear cross roller bearings 220 and 222 preventthe inner motion frame 214 from rotating relative to the external screwshaft 210 and the internal screw cylinder 212. Field 234 and armature236 of the prime mover 202 supported by bearings 238 and 240 drive thelinear planetary screws 216 in spindle bearings 244 in spindle cage 246supported by principal thrust bearings 248. Field 250 and armature 252of the prime mover supported by bearings 254 drive the linear planetaryscrews 218 in spindle bearings 240 in spindle cage 258 supported byprincipal thrust bearings 260.

Note that only one set of the linear cross roller bearings 220 and 222is necessary to constrain the rotary motion of the inner frame 214.Bearing 230 is more effective in resisting the torque load on the innerframe 214 because of the larger diameter and higher torsional stiffnessof the outer cylinder shell 226.

The linear fault-tolerant actuator 200 of FIG. 2 is not only faulttolerant in velocity summing between two independent prime movers 202and 204, but also exhibits no single point failures between its twolinear screw transmissions. This is a velocity summing concept withreconfiguration of the prime mover velocities in real time. The designin FIG. 2 has considerable merit for applications requiring compactness,greater simplicity, higher ruggedness, and partial fault tolerance inthe electrical prime movers and their electronic control subsystems.

Many applications require a combination of low output velocity and highoutput force. Also, desirable properties of small size, high stiffness,and low cost usually accompany this type of application. FIG. 3 depictsa linear actuator module 300 that uses two externally-threaded rotaryactuators 302 and 304 to drive two internally-threaded cylinders 306 and308 in series. In certain embodiments, module 300 may be designed togenerate high force at relatively low cost. Although not necessarilyoptimized for applications requiring high linear velocities or rapidresponse to input commands, module 300 may be optimized to generate highforce in a rigid, yet small package. In certain embodiments, three ormore linear actuators (e.g., 302-304) may be combined to create an evenmore fault-tolerant linear actuator module 300.

In module 300 there is one external rectangular cylinder 310 attached tothe actuator reference frame 312. Actuator reference frame 312 anchorseach of the (externally-threaded) internal rotary actuator modules 302and 304. In certain embodiments, the two internally-threaded rectangularcylinders 306 and 308 use linear cross roller bearings 314 and 316 forprecision and stiff operation relative to the external rectangularcylinder 310. Other embodiments may employ sleeve-type bearings for thesame function.

Module 300 may be employed in very low cost applications, such as inautomobiles or in very low weight applications, as found in thedeployment of large flaps on aircraft. In a manufacturing cell, module300 may also be used in fixturing. Combined with high precision smallmotion actuators, module 300 is useful for application where both veryhigh force and high precision are required.

The threaded interface between the externally-threaded rotary actuators302 and 304 and the internally-threaded rectangular cylinders 306 and308 may vary by application. For example, certain embodiments employacme screw thread. Acme screw mechanisms are low in cost, resistant toshock and oscillatory forces, tolerant of contamination, and reliablefor extended service at low velocities. Acme threads will, however,generate more friction than alternate transmissions such as the ballscrew or the spindle screw.

FIG. 4 depicts a linear fault tolerant actuator 400 having no singlepoint failures. This is achieved by creating dual force paths in asingle envelope wherein either of the force paths (prime mover andtransmission) may be removed from service by a clutch release or similarmechanism in the event of failure.

FIG. 4 depicts an isometric cutaway of a dual force path linear actuator400. The system uses a pair of planetary roller screws 402 and 404driven by separate prime movers 406 and 408, all in a concentricconfiguration. Prime mover 406 drives planetary roller screws 402, whichin turn drive a roller screw shaft 414 with external threads. Primemover 408 drives planetary screws 404 that drive a roller screw cylinder420 with internal threads. The roller screw shaft 414 and the rollerscrew cylinder 420 are attached at one end to an output cylinder 422.

The roller screw cylinder 420 is separated from the output cylinder 422by the outer clutch 424, while the roller screw shaft 414 is separatedfrom the output cylinder 422 by the inner clutch 426. Should either ofprime movers 406 or 408 fail, the associated clutch 424 or 426 may beenergized to take that prime mover 406 or 408 out of service. Thissystem ensures that operation would continue even under a major fault inone of the force pathways. In certain embodiments, a single force pathmay have the capacity to double its normal output for a short period oftime to compensate for the failed subsystem, in order to prevent anymajor system failure.

Roller screw shaft 414 and outer shell, along with the roller screwcylinder 420, are connected through clutches 424 and 426 to the outputcylinder 422, by means of end cap screws 428. Nut 430 connects the screwshaft 414 to the plate 432, which holds inner clutch 426.

As noted above, there are two separate prime movers 406 and 408 withinlinear actuator 400. Field 434 and armature 436 on support bearings 442drive planetary screws 402 supported by spindle bearings 444. Spindlebearings 444 transfer forces through the planetary screw cage 446 toprincipal thrust bearing 448 to the inner motor frame 450 holding themotor fields, which is attached to the input attachment cylinder 452through end cap screws 428.

The second prime mover 408 incorporates field 438 and armature 440 onsupport bearings 454 driving planetary screws 404 through supportbearings 456. Support bearings 456 act through the planet cage 458 bymeans of thrust bearings 460. Hence, each prime mover-transmissioncombination independently creates a driving force on the output cylinder422.

Constructed as shown in FIG. 4 and described above, linear actuator 400eliminates the risk of total actuator failure brought on by any singlepoint failure. Failures associated with threat to life, a significanteconomic loss, or the continuation of a long duration mission allsuggest the need for continued operation even under a fault such as alost prime mover, transmission, communication link, sensor, or powersupply. Achievement of this goal requires the inclusion at least twofully independent pathways to drive the output. In the past, this meantthat two separate linear actuators were arranged side-by-side and set upwith separate control loops.

Although the inclusion of a separate actuation mechanism provides for adegree of fault tolerance, such a combination is complex,space-inefficient and heavy. Such a design also introduces a level offunctional uncertainty that designers find unattractive. Redundancy,which sets aside one part of a dual system while the other one operatesis a waste of both resources and priorities (weight, volume, cost,etc.).

In the embodiment shown in FIG. 4, all resources are employed at alltimes, maximizing output performance and accepting a reduced performancereserve in the event of a partial fault.

A fifth embodiment of the present invention is shown in FIG. 5 andgenerally designated 500. Actuator 500 is made up of two completelyindependent subsystems 502 and 504 to provide operation even under acomplete failure of one of the subsystems.

The two actuator subsystems 502 and 504 of actuator 500 are geometricinverses of each other. Spindle screw set 522 drives a small diameterscrew shaft 534 with external threads, while spindle screw set 538drives a large diameter screw cylinder 540 with internal threads.Spindle screw set 538 may be at a diameter three times greater thanspindle screw set 522, which would, of course, require an angularvelocity reduction of three-to-one in order to maintain the same contactlinear velocity at the screw threads. This reduction also reduces thestored kinetic energy in the rotating parts.

Subsystem 502 is driven by prime mover 508. Subsystem 502 is guided andsupported by cage 512, which holds planet gears 514 in planet bearings516. Planet gears 514 mesh with bull gear 518 and sun gear 520, thatdrive sun gear 520 attached to the spindle screw set 522 supported byspindle nut support bearings 524. The principal cross roller bearing 526separates the sun gear 520 from the bull gear 518 and transfers theactuator load from the spindle set 522 to the actuator carriage at thebull gear 518. End caps 528, 530, 532 are used to assemble subsystem502.

Subsystem 504 may be described in the same manner as subsystem 502,except that it is the geometric inverse of subsystem 502. In operation,axial loads pass from the actuator screw shaft 534 to spindle screw set522 through principal cross roller bearing 526 to the actuator carriage554 and then through principal cross roller bearing 536 on to spindlescrew set 538 out to the outer shell 540 of the actuator 500. Theanti-rotation splines 542 and tangs 544 prevent the carriage fromrotating in the actuator 500. Seals 546 and 548 prevent the escape ofthe lubricant from the actuator 500. A utility coil volume 550 isprovided between the actuator carriage and the end-cap 552 of the outercylinder shell 540 for the supply of power, communications, andlubricant to the moving carriage.

In special applications, the need for low weight is critical. This mayachieved, for example, by using high RPM prime movers. There becomes amismatch between this high RPM and the low speed/high force needed atthe output shaft. To make this combination feasible, an intermediategear reduction must occur between the motors and the linear spindlescrew transmissions.

In normal prime mover applications, a prime mover maximum angularvelocity between at least about 3,000 and 4,000 RPM is generallyconsidered ideal. For extremely low-weight applications, maximum primemover angular velocities between at least about 15,000 and 30,000 RPMmay be required. Such designs may output five to ten times morehorsepower for the same weight of the prime mover. In order to multiplythe motor torque, a first stage gear reducer, such as an epicyclic geartrain, is inserted between the prime movers and the associated linearspindle screw transmission in order to balance the input and outputspeeds, as well as the forces involved. This first stage reductionallows for design optimization of both the prime movers and the linearspindle screw transmission.

Structurally, the strength of actuator 500 is entirely dependent on theload carrying capacity of the spindle screw sets 522 and 538 and the twoprincipal cross roller bearings 526 and 536. Subsystem 502, whichincludes spindle screw set 522, crossroller bearing 520, geartransmission 514 and prime mover 508, is completely independent ofsubsystem 508, but they occupy a common moving carriage, which transfersthe load from the actuator screw shaft to the outer cylinder screwshell.

Because the spindle screw sets 522 and 538 create a turning resistancedue to friction, an anti-rotation spline 542 is built into the rightside of the actuator screw shaft 534, in order to prevent rotation ofthe carriage 554. In one embodiment, it is likely that spindle screwsets 522 and 538 will be of the same length to carry the same load.

In another embodiment, the lead on spindle screw set 522 is at leastabout 0.2 in./rev. given a desired output speed of at least about 3.5in./sec., an angular velocity of at least about 1050 RPM would bedemanded of prime mover 508. The intermediate gear transmission ratiofor subsystem 502 would have to be at least about 14.3 to 1. Theequivalent desired speed for spindle set 538 would be at least about 300RPM.

In yet another embodiment, the lead of the internal cylinder screw 550is at least about 0.7 in./rev. Given a maximum angular speed of at leastabout 30,000 RPM for second prime mover 556, the intermediate geartransmission ratio of subsystem 504 would be at least about 100-to-1.The low speeds in the spindle screws 522 and 538 will be very helpful inextending the life of these critical parts in actuator 500.

Nonetheless, the high rotational speed requirements place considerabledemands on the intermediate gear transmissions. First, the exceptionallyhigh angular velocities will store considerable kinetic energy. Forepicyclic gears, this requires that the planets be as small as possible.

In certain additional embodiments, the subsystems 502 and 504 mayoperate in opposite directions in order to better balance the frictionturning torques on the moving carriage 554.

Owing to the use of roller screws, subsystems 502 and 504 are naturallynon-backdrivable. Depending on the pitch of the screw threads and theapplication, there still may be a need to put in place brakes on each ofthe armatures to prevent the system from walking under oscillatingexternal loads.

In certain other embodiments of actuator 500, each subsystem 502 and 504provides one-half of the total stroke length. Accordingly, actuator 500may always return to the neutral position and operate in only one-halfits useful range, with one side completely incapacitated. Alternately, apartially failed side could “limp” home to the center of its range, andthen be locked in place, so that the remaining operable side couldprovide fifty percent of the range capacity about the center position.

It should be mentioned that in some applications, it would be useful toprovide for consistent lubrication of the actuator. For example, a lowviscosity oil under pressure may be used to provide a misted atmosphereinside the actuator volume. The lubricant could be recirculated in aclosed circuit and may also be cooled if the duty cycle demands thatheat be removed from the system. This, then, requires at least about twoseals: a first seal between a smooth surface on the carriage 554 and theouter cylinder shell 540 and a second seal between a smooth surface onthe actuator screw shaft 534 and an extension of the actuator carriage540. The other end of actuator 500 is sealed by an end cap 552 on theouter cylinder shell 540.

Additional objects, advantages and novel features of the invention asset forth in the description that follows, will be apparent to oneskilled in the art after reading the foregoing detailed description ormay be learned by practice of the invention. The objects and advantagesof the invention may be realized and attained by means of theinstruments and combinations particularly pointed out here.

1. A linear actuator comprising: a substantially-cylindrical actuatorframe having a principal axis; a linear output screw having threadsdisposed thereon, the linear output screw disposed at least partlywithin the actuator frame along the principal axis; a planetary spindlescrew set, having threads mated to the threads of the linear outputscrew, disposed in a spindle screw carrier about the linear outputscrew; a first armature disposed within the actuator frame about theprincipal axis; a second armature disposed within the actuator frameabout the principal axis; a differential gearset disposed in the spindlescrew carrier, and connected to the first and second armature in suchmanner to provide a differential action between the first and secondarmature.
 2. The actuator of claim 1 further comprising a first fieldcoil cylinder disposed around the first armature and a second field coilcylinder disposed around the second armature.
 3. The actuator of claim 1wherein the first armature is rotationally fixed to a first gear on oneend of the first armature meshed to the differential gearset and thesecond armature is rotationally fixed to a second gear on one end of thesecond armature meshed to the differential gearset.
 4. The actuator ofclaim 1 wherein the differential gearset comprises four centraldifferential planetary gears.
 5. The actuator of claim 1 furthercomprising a set of sensors for constantly monitoring the torque outputof each armature.
 6. The actuator of claim 1 wherein the differentialgearset is supported by a differential cage.
 7. The actuator of claim 1wherein the set of planetary spindle screws comprises a first set ofplanetary spindle screws at a first axial location along the linearoutput screw and a second set of planetary spindle screws at a secondaxial location along the linear output screw.
 8. The actuator of claim 1wherein the actuator frame further comprises a trunnion extendingradially from the principal axis of the actuator frame.
 9. The actuatorof claim 1 further comprising a first brake to lock the radial positionof the first armature with respect to the actuator frame and a secondbrake to lock the radial position of the second armature with respect tothe actuator frame. 10-19. (canceled)
 20. A linear actuator comprising:an actuator frame having a central reference frame fixed thereto; afirst rotary actuator having a fixed portion that is fixed to thecentral reference frame, and a radially-movable portion; a second rotaryactuator having a fixed portion that is fixed to the central referenceframe, and a radially-movable portion; a first cylinder threadablyengaged with the radially-movable portion of the first rotary actuator;and a second cylinder threadably engaged with the radially-movableportion of the second rotary actuator.
 21. The actuator of claim 20wherein the first and second cylinders interact with theradially-movable portions of the rotary actuators through acme threads.22. The actuator of claim 20 wherein the first and second cylindersinteract with the radially-movable portions of the rotary actuatorsthrough ball screws.
 23. The actuator of claim 20 wherein the first andsecond cylinders interact with the radially-movable portions of therotary actuators through roller screws.
 24. The actuator of claim 20wherein the central reference frame has a first side and a second sideopposite the first side, and wherein the first actuator is disposed onthe first side and the second actuator is disposed on the second side.25. The actuator of claim 23 wherein the first cylinder is disposed onthe first side and the second cylinder is disposed on the second side.26. The actuator of claim 20 wherein the first and second cylinders havea rectangular outer cross-section.
 27. The actuator of claim 20 whereinthe first and second cylinders are axially-movable and radially-fixedwith respect to the central reference frame.
 28. The actuator of claim20 further comprising an external rectangular frame fixed to the centralreference frame.
 29. The actuator of claim 28 wherein the first andsecond cylinders are suspended in the external rectangular frame by aset of roller bearings. 30-39. (canceled)
 40. A linear actuatorcomprising: a substantially-cylindrical actuator frame having aprincipal axis, an electromagnetic field therein, and an internalsurface having a thread disposed thereon; a linear output screw havingthreads disposed thereon, the linear output screw disposed at leastpartly within the actuator frame along the principal axis; a firstarmature disposed within the actuator frame about the principal axis; afirst spindle screw set, having threads mated to the threads of thelinear output screw, disposed in a first carrier about the linear outputscrew; a first geartrain, disposed between the first armature and thefirst carrier, connecting the first armature and the first carrier insuch a manner as to maintain a fixed ratio between the angular velocityof the first armature with respect to the actuator frame and the angularvelocity of the first carrier with respect to the actuator frame; asecond armature disposed within the actuator frame about the principalaxis; and a second spindle screw set, having threads mated to theinternal threads of the actuator frame, disposed in a second carrierabout the linear output screw; and a second geartrain, disposed betweenthe second armature and the second carrier, connecting the secondarmature and the second carrier in such a manner as to maintain a fixedratio between the angular velocity of the second armature with respectto the actuator frame and the angular velocity of the second carrierwith respect to the actuator frame.
 41. The actuator of claim 40 furthercomprising a first field coil cylinder axially aligned with the firstarmature and a second field coil cylinder axially aligned with thesecond armature.
 42. The actuator of claim 41 wherein the first andsecond field coil cylinders are disposed in a movable carriage.
 43. Theactuator of claim 42 wherein the movable carriage is axially-movablewith respect to the actuator frame, but is radially-fixed with respectto the actuator frame.
 44. The actuator of claim 43 wherein the movablecarriage comprises a set of bearings for supporting the first and secondgeartrains.
 45. The actuator of claim 44 further comprising a movablecarriage axially-movable with respect to the actuator frame, butradially-fixed with respect to the actuator frame.
 46. The actuator ofclaim 45 wherein the first planetary spindle screw set is axially-fixedwithin the first carrier but radially-movable, and wherein the secondplanetary spindle screw set is axially-fixed within the second carrierbut radially-movable.
 47. The actuator of claim 40 further comprising aset of sensors for constantly monitoring the torque output of eacharmature.
 48. The actuator of claim 40 wherein the first set ofplanetary spindle screws are disposed at a first axial location alongthe linear output screw and the second set of planetary spindle screwsare disposed at a second axial location along the linear output screw.49. The actuator of claim 40 further comprising a first brake to lockthe radial position of the first armature with respect to the carriageand a second brake to lock the radial position of the second armaturewith respect to the carriage.